My shop is 20x20 and it is just right for me. That space is dedicated to woodworking. I store my wood outside and have enough storage inside for the project I'm working on. My plans include a future room for finishing and another to hold the cyclone. This still leaves me with my 20x20 workspace.
I know the common wisdom is to build as large as you can. When I built my stand alone shop, I decided the more room, the more clutter and a lot of pack-rattiness. My shop is ideal for me, and I build some pretty large things like the 15' sailboat presently occupying the shop.
John